(1) Field of the Invention
The present invention relates to a stepping motor drive device. More particularly, the present invention relates to a technique of driving a stepping motor with low noise, low vibration, and high efficiency.
(2) Description of the Related Art
Because of its suitable performance characteristic for positioning control, stepping motors have been conventionally used in photo graphing electronics devices, such as a DSC (Digital Still Camera or so-called “digital camera”) and a DVC (Digital Video Camera) for adjustment of aperture, focus, zoom, and the like.
Stepping motors, especially those used in photographing electronics devices, are required to operate with low noise and low vibration. This is because sounds generated by a stepping motor are caught by a built-in microphone of a photographing electronics device and recorded as noise, and vibrations generated by a stepping motor cause unsteadiness of a photographing electronics device and result in degradation in picture quality.
In response to the above need, a technique of driving a stepping motor with low noise and low vibration is disclosed, for instance, in Japanese Patent Application Publication No. H06-343295.
FIG. 21 shows a drive device disclosed in the publication. (The following description focuses only on components that are necessary for explaining the principle of the device.)
In FIG. 21, the reference numeral 20 denotes a stepping motor that is driven by the drive device. The stepping motor 20 includes a rotor 45, a first coil 19a, and a second coil 19b. 
An up-down counter 43a counts up or down a clock signal CLKP in accordance with an up-down signal DA, and outputs 4-bit signals DA1-DA4 representing the count to a D/A converter 44a. 
The D/A converter 44a outputs a voltage signal VCA corresponding to the 4-bit signals DA1-DA4. The voltage signal VCA has a staircase waveform that rises and falls stepwise. The rapidity of change in level of the voltage signal VCA is variable depending on a frequency of the clock signal CLKP (a pulse frequency, to be more precise), as well as an amount of decrement or increment of the voltage signal VCA in each step. When the pulse frequency of clock signal CLKP is higher, as well as when the amount of decrement or increment of voltage signal VCA in each step is greater, the rate of change in level of the voltage signal VCA is greater. Further, by stopping the clock signal CLKP, the voltage signal VCA can be held at a fixed level.
A drive circuit 39a amplifies the voltage signal VCA by a non-inverting power amplifier 41a and also by an inverting power amplifier 42a. Thus, the first coil 19a connected between respective output terminals of the power amplifiers 41a and 42a is driven through the application of a voltage.
The same set of components as described above are provided for the second coil 19b too, to drive the second coil 19b through the application of a voltage based on a staircase waveform.
According to this construction, a voltage applied to a coil increases stepwise at the beginning of energization and decreases stepwise at the end of energization, so that abrupt torque fluctuations at the beginning and end of energization are reduced. Consequently, there is an effect of reducing vibration and noise that are generated by the torque fluctuations compared with the case where a voltage applied to a coil rises and falls in one stroke.
Unfortunately, however, the above conventional drive device is incapable of decreasing the coil current promptly enough upon the step-down of the staircase waveform, and thus the coil current fails to accurately follow the staircase waveform. Due to this insufficient accuracy, even if a voltage signal approximates a staircase waveform suitable for suppressing noise and vibration, e.g. a sinusoidal waveform, the coil current fails to closely follow the approximately sinusoidal staircase waveform. This gives a rise to a first problem that an intended effect of noise and vibration suppression is not achieved.
In addition, there is a second problem that the conventional drive device is power-consuming because the coil current is supplied at all times.